Singularize method for Rails String class

string.rb

# encoding: UTF-8

class String
  def singularize_es
    if self[-2..-1].index(/es/)                   # Ending in 'es'
      if self[-4..-1].index(/[^aeiou][^aeiou]es/) # Ending in consonant + consonant + 'es': remove 's' ('coche' > 'coches')
        self[0..-2]
      else
        if self[-3..-1].index(/[^aeiou]es/)       # Ending in consonant + 'es' 
          case self[-3..-1]
          when 'nes'                              # Ending in 'nes': add accent to precedent vowel ('camiones' > 'camión')
            replacements = {'a' => 'á', 'e' => 'é', 'i' => 'í', 'o' => 'ó', 'u' => 'ú'}
            self[-4..-4] = replacements[self[-4..-4]]
            self[0..-3]
          when 'ces'                              # Ending in 'ces': replace with 'z' ('peces' > 'pez')
            self[0..-4] + 'z'
          else                                    # General case: remove 'es' ('pieles' > 'piel')
            self[0..-3]
          end
        else                                      # Ending in vowel + 'es': remove 's' ('pies' > 'pie')
        	self[0..-2]
        end
      end
    else
      if self[-2..-1].index(/s/)                  # Ending in 's'
        self[0..-2]
      else
        self
      end
    end
  end
end